1. Field of the Invention
The present invention relates to a metallic separator of a fuel cell workable at a relatively low temperature, such as a solid macromolecular fuel cell, and a manufacturing method thereof.
2. Background Information
A solid macromolecular fuel cell has such advantages that it works at a temperature below 100xc2x0 C. and that it starts working in a short time, compared with other types of fuel cells. Since such the fuel cell has a structure composed of all solid members, it is easily maintained in an operable state and applicable for various uses subjected to vibrations or impacts. In addition, the fuel cell can be designed to a small size due to high power density. The fuel cell also has good fuel efficiency with less noise. Accounting these advantages, application of the fuel cell to a motor installed in an electric automobile or the like has been researched and examined so far. If fuel cells which cover a long travelling distance similar to that of a gasoline engine is provided, an automobile which installs such fuel cells therein does not substantially put any harmful influences on the environment due to no generation of SOx or NOx with a reduction of CO2 by half.
A conventional solid macromolecular fuel cell has a solid macromolecular membrane containing a proton-exchanging group in its molecular structure. The membrane acts as a proton-conductive electrolyte. An interior of the fuel cell is divided into two zones by the membrane. A fuel gas such as hydrogen is supplied to one of the zones, while an oxidizing gas such as oxygen is supplied to the other zone, in the same manner as other types of fuel cells.
The fuel cell has an inner structure schematically illustrated in FIGS. 1A and 1B. An air electrode 2 and a hydrogen electrode 3 are coupled to both sides of a solid macromolecular membrane 1, respectively. Both sides of the membrane 1 are faced through gaskets 4 to separators 5. An air-supply hole 6 and an air-discharge hole 7 are formed in the separator 5 at the side of the air electrode 2, while a hydrogen-supply hole 8 and a hydrogen-discharge hole 9 are formed in the other separator 5 at the side of the hydrogen electrode 3.
A plurality of grooves 10 which extend along flowing directions of hydrogen g and oxygen or air o are formed in the separators 5, in order to uniformly distribute hydrogen g and oxygen or air o. Cooling water w is fed through water-supply holes 11, circulated in the separators 5 and discharged through water-discharge holes 12 by water-cooling means provided in the separators 5, so as to release a heat during power generation.
Hydrogen g, which is fed through the hydrogen-supply hole 8 to a gap between the hydrogen electrode 3 and the separator 5, converts to a proton after discharge of an electron. The generated proton permeates through the solid macromolecular membrane 1, accepts an electron at the side of the air electrode 2, and burns with oxygen or air o which passes through a gap between the air electrode 2 and the separator 5. Consequently, an electric power is gained by charging a load between the air electrode 2 and the hydrogen electrode 3.
Since an electro motive force per one fuel cell is very tiny, a plurality of fuel cells is laminated together to gain a voltage necessary for practical use, as shown in FIG. 1B. Herein, a solid macromolecular membrane sandwiched between separators is handled as one unit. Due to the constitution that a plurality of fuel cells is laminated together, power-generating efficiency is significantly affected by resistance of the separators 5. A separator material having good electric conductivity with low contact resistance is necessary for improvement of power-generating efficiency. In this regard, graphite separators have been used so far with the same idea as that for a phosphate fuel cell, as disclosed in OHM Vol. 83, No. 7, pp. 55-61, and FUJI JIHOH Vol. 68, No. 3, pp. 164-167.
Such a graphite separator is offered by cutting a graphite block to an objective shape and machining the shaped graphite block to form various holes and grooves. The cutting-machining process excessively consumes graphite material and needs expensive processing fees, so that a fuel cell as a whole is very expensive. The cutting-machining process is also inferior of productivity. Besides, a separator made of brittle graphite is easily broken or damaged by vibrations, impacts and so on. In order to overcome these disadvantages of a graphite separator, JP8-180883 A1 proposed a method of manufacturing a separator from a metal sheet by pressing, punching and so on.
However, when a metal sheet is used as a material for a separator of a fuel cell, there appears another problem. That is, a zone at a side of the air electrode 2 for passage of oxygen or air o is an acid atmosphere with pH 2-3. There has not been developed a metallic material, which sufficiently endures in a strong acid atmosphere and exhibits properties necessary for use as a separator, e.g. superior electric conductivity, low contact resistance with electrodes and corrosion resistance.
An acid-resistant material such as stainless steel could be used as a metallic material endurable in an acid atmosphere. Such a material exhibits excellent acid-resistance due to a passivated layer formed on its surface, but the passivated layer raises surface or contact electric resistance of the material with hydrogen and air electrodes. Elevation of the contact resistance means generation of a big quantity of a Joule heat at contact planes of separators to the hydrogen and air electrodes. Generation of the Joule heat causes wasteful consumption of an electric power gained by fuel cells, resulting in decrease of power-generating efficiency. Other metal sheets ordinarily also have oxide layers, which raise contact resistance, thereon.
Au is a metal material which does not have a passivated or oxide layer on its surface, and endurable in an acid atmosphere. However, Au is a very expensive material, so that it can not be practically used as a proper material for a separator of a fuel cell. Pt is also a metal material which is resistant to formation of a passivated or oxide layer on its surface and endurable in an acid atmosphere. However, Pt can not be used as a separator material due to its expensiveness.
In addition, a metal material for use as a separator shall be good of workability, since a plurality of grooves 10 or flanges for passages of hydrogen and air are formed by pressing, punching and so on. Workability of the metal material could be improved by applying an organic macromolecular film or a lubricating agent onto a surface of the metal material. However, application of an organic macromolecular film or lubricating agent raises contact resistance of the metal material, so that a large quantity of a Joule heat would be generated in a power generator having a plurality of fuel cells laminated. Generation of a Joule heat means a loss of an electric power and reduces a power-generating efficiency of the power generator.
After a metal material to which a lubricating agent was applied is worked to an objective shape, the metal material shall be subjected to post-treatment such as degreasing and rinsing. Such post-treatment means an increase of processing steps, and also needs great expenditures for treatment of waste liquids. If the worked metal material is degreased using an organic or flon solvent, the atmosphere would be deteriorated by diffusion of the solvent. When an organic film is applied onto a surface of a metal material, the metal material can be worked to an objective shape without use of a lubricating agent. However, contact resistance of the metal material is raised by the applied organic film, and also the organic film is peeled off or dissolved away from a surface of the metal material due to its poor endurance in an acidic atmosphere.
The present invention is a metallic separator which eliminates above-mentioned problems. Excellent electric conductivity and low contact resistance of the metallic separator is ensured without decrease of acid resistance by dotted distribution of carbonaceous particles on a surface of a stainless steel or formation of a metal plating layer or a paint film, in which carbonaceous particles are dispersed, on a surface of a stainless steel.
A first-type separator for a low-temperature fuel cell according to the present invention includes adhesion of carbonaceous particles onto a surface of a separator made from a corrosion-resistant metal sheet which has an oxide layer preformed in a corrosive atmosphere. A representative metal sheet as a substrate is a stainless steel having a passivated layer on its surface. Carbonaceous particles are preferably applied onto the surface of the substrate sheet with dotted distribution.
Carbonaceous particles are pressed onto a stainless steel sheet by applying the carbonaceous particles onto a surface of the stainless steel sheet and then rolling the stainless steel sheet with a reduction ratio of approximately 0.1-50%, to improve adhesiveness and peeling-resistance of carbonaceous particles onto the stainless steel substrate. The stainless steel sheet may be heat-treated after pressing the carbonaceous particles. A diffusion layer effective for adhesiveness is formed between the carbonaceous particles and the stainless steel substrate by the heat treatment. The carbonaceous particles may be carbon black or graphite particles.
A second-type separator has a stainless steel substrate coated with a metal plating layer in which carbonaceous particles are dispersed in a state exposed to the atmosphere. The plating layer may be a Nixe2x80x94Cr, Ti, Ta or Tixe2x80x94Ta layer. The carbonaceous particles to be dispersed in the plating layer may be carbon black or graphite particles. The Nixe2x80x94Cr plating layer preferably contains approximately 5-60 wt. % Cr and optionally approximately 0.3-40 wt. % Mo.
A third-type separator has a carbon-bonded layer composed of carbonaceous particles bonded through a diffusion layer onto a surface of a stainless steel substrate. Fine granular carbon adheres onto surfaces of the carbonaceous particles in the carbon-bonded layer. The carbon-bonded layer can be formed by applying a carbonaceous particle-dispersed paint onto the stainless steel substrate, and then decomposing and vanishing organic components with a heat to retain the carbonaceous particles on the surface of the stainless steel substrate. Thermal decomposition of the paint film may be performed by heat-treatment at about 300-1150xc2x0 C. in a non-oxidizing atmosphere. Before the heat-treatment, the stainless steel sheet coated with the paint film may be rolled with a reduction ratio of 0.1-50%.